1. Field of the Invention
The present invention relates to an imaging device and imaging method, in an imaging device such as a digital camera, capable of shooting a high brightness subject, and having a high degree of freedom for exposure conditions in order to obtain optimum exposure.
2. Description of the Related Art
In an imaging device such as a conventional digital camera, exposure control is carried out by changing aperture value, ISO sensitivity, shutter speed etc. A setting range for aperture value (generally represented by F number) is restricted to a limited range in order to maintain optical characteristics, such as lens MTF (Modulation Transfer Function: value relating to resolution). Lenses currently on the market generally have a range of about F1.0 to F22.
Also, a low sensitivity limit and a high sensitivity limit of ISO sensitivity is determined by characteristics of an image sensor, such as a CMOS image sensor or a CCD image sensor, and permissible image quality depending on camera specification.
In the case where a mechanical shutter has been adopted as the shutter, then from the viewpoint of the setting range for shutter speed (generally, represented as Tv value relating to the reciprocal of exposure time), with a shutter speed that gives a longer exposure time (small Tv value), dark current noise of the image sensor increases with longer shutter speed. Therefore, image quality is degraded by increasing dark current noise, and permissible time for shutter speed at a low speed end of the range is determined by a value relating to S/N ratio for image quality (value using ratio of correct signal components and noise components in an image signal) similar to the high sensitivity setting for ISO sensitivity. On the other hand, a setting range for shutter speed at the high-speed end is dependent on drive precision of a mechanical shutter. For example, in the case of a focal plane shutter, since there is variation in opening and closing speed of the two shutter curtains, namely the front curtain and the rear curtain, shutter speeds are currently limited to about 1/8000th of a second. Also, in the case where a lens shutter has been adopted as the shutter, the limit speed at the high-speed end is lower than with a focal plane shutter.
As mentioned above, there are respective limits for aperture value, ISO sensitivity and shutter speed for controlling exposure, and with combinations of aperture value, ISO sensitivity and shutter speed within the limit range there may be situations where a user, being a photographer, can not obtain the image they intended. Basically, in the case of shooting an image of high brightness, such as the sun, even if the aperture is closed down to F22 and ISO sensitivity set to the lowest sensitivity, and shutter speed set to the highest speed, there may be situations where the taken image still has blown out highlights.
In the case of shooting this type of high brightness subject, the conventional approach has been to adopt an ND (neutral density) filter, and shoot with correct exposure by reducing light amount incident on the image sensor. Since fitting an ND filter to a camera lens is troublesome, there has also been proposed a digital camera that has an ND filter built into an incident light path within the camera, and is capable of inserting or retracting the ND filter in front of the image sensor as required (refer to Japanese patent laid-open No. 2003-283923 (hereafter referred to as patent publication 1)). There has also been proposed an image sensor that is capable of shooting with effectively lowered sensitivity, by eliminating some electrical charge signals that have been photoelectrically converted by photodiodes of the image sensor, in order to carry out shooting with effectively reduced sensitivity without using an ND filter (refer to Japanese patent laid-open number 2013-183380 (hereafter referred to as patent publication 2)).